Miniaturized tem microwave bandpass filter



Oct. 20, 1964 E. w. SEELEY ETAL 3,153,768

MINIATURIZED TEM MICROWAVE BANDPASS FILTER Filed March 29, 1962 CENTRAL CONDUCTOR ELWIN s-EELEY JOHN ALDAY INVENTORS ATTORNEYS United States Patent Oil ice 3,153,768 Patented Oct. 20, 1964 3,153,768 ETTNTATURZZED TEM MTCROWAVE BANBPASS FILTER Elwin W. Seeley, fiverside, and John R. Aiday, Corona,

Caliii, assignors to the United States oi America as represented by the Secretary of the Navy Fiied Mar. 29, 1962, Ser. No. 183,683 3 Claims. (Cl. 333-73) (Granted under Title 35, US. Code (1952 sec. 266) The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to waveguide filters and particularly to Waveguide filters for use in the microwave region below X-band.

While most filter problems in the microwave region can be handled by common waveguide there are applications where coaxial waveguide filters may be necessary due to size and weight limitations or for use with frequencies below X-band, for instance.

It is an object of the present invention to provide a novel TEM bandpass microwave filter.

It is also an object of the invention to provide a miniature ruggedized coaxial microwave filter of simple design and construction.

Another object of the invention is to provide an improved and more efiicient coaxial TEM bandpass microwave filter.

A further object of the invention is to provide a novel microwave filter for use with rectangular, square or elliptical coaxial waveguide.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein the figure of the drawing illustrates a microwave filter of the present invention constructed in a section of rectangular waveguide.

As shown in the figure of the drawing, the entire filter is enclosed in ordinary size rectangular waveguide having a conductor 12 which runs through the geometric center of the waveguide. A filter of the present invention constructed in rectangular waveguide is described herein, by way of example; however, this type filter may also be constructed in square or elliptical coaxial waveguide.

The filter consists basically of inductive stubs placed in tandem along a length of transmission line. In constructing a filter, using a rectangular waveguide as illustrated in the drawing, the narrow sides 14 of waveguide 16 are of substantial thickness whereas the broad sides are of standard thickness. Cavities 16, 17, 18 and 19 are formed in each of the walls 14 of the waveguide; the depth of these cavities vary in accordance with the length of the inductive stubs. The inductive stubs are each comprised of a cavity and a conductor mounted normal to and connecting central conductor 12 with the bottom of the cavity. Conductors 21, 22, 23, 24, 25, 26, 27 and 28 together with their respective cavities each form individual stubs. Each pair of opposite stubs form one element.

The inductive reactance of each stub, can be varied by varying its length d or the width w of its cavity. The depth d of the stubs and the separation S between stubs or elements are determined by mathematical formulas and depend upon the frequency of operation, bandwidth and selectivity required by the application of the filter. Mathematical formulas are found in A Simplified Approach to the Design of Bandpass Filters in Waveguide, by E. S. Hensperger, Microwave Journal, vol. 2, No. 11, November 11, 1959. However, in such formulas, a mathematical substitution of the free space wavelength is made for the waveguide wavelength to set the formulas in the TEM mode of transmission.

It takes a minimum of two elements to form a filter; two elements are necessary to form a one section filter, three elements form a two section filter, etc. The figure of the drawing illustrates a three section filter and therefore has four elements comprised of four parallel pairs of stubs. In the example shown for a three section filter, the element depth (D of the first and fourth elements are equal, the depth (d of cavities 15 and 19 being equal, and the depth (D of the second and third elements are equal, the depth (d of cavities i7 and 18 being equal. However, the depth of the first element (D should not equal the depth of the second element (D Also, the separation, S, between filter sections varies; that is, the separation (S between the first and second elements is equal to the separation (S between the third and fourth elements, but the separation (S between the second and third elements is not equal to the separation S or The filter of the present invention permits the transmission of a certain band of frequencies. The characteristics of this bandpass is determined by the physical dimensions of the stubs. The center of the pass band is determined by the distance (5;, S S etc.) between elements. The skirts of the pass band response curve will be determined by the depth (d d etc.) of the stubs and by the number of elements.

As can be seen from the figure, there are certain inherent limitations in the filter due to the limited range of individual stub reactance. The limits occur when the depth of the stub :1 equals a, where a is the half-width of the waveguide as shown in the drawing, and when d equals b, where b is the distance between the coaxial conductor 12 and the inside of the thick walls of the waveguide. These limitations are important when the design of very broad or very narrow bandpass filters is considered. For very narrow filters [1 must be very small, thus limiting the power capabilities of the filter. For very broad filters a must be nearly a quarter wavelength long, thus increasing the filter size.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A coaxial waveguide miniature TEM microwave bandpass filter comprising:

(a) a section of waveguide having a coaxial conductor which runs therethrough the walls of the broad sides of said section of waveguide being of standard thickness and the walls of the narrow sides of said section of waveguide being of substantial thickness,

(b) a plurality of inductive stubs positioned in tandem along the length of said waveguide and in electrical contact with said coaxial conductor, each pair of stubs in tandem forming one element, each inductive stub consisting of a cavity formed in a thick wall of said waveguide section and confined within said thick wall with a stub conductor therein mounted normal to and connecting said coaxial conductor and the end of said cavity, said cavities being entirely within the outer walls of said waveguide section,

(0) the center of the pass band being determined by the distance between elements along the length of the section of waveguide,

(d) the characteristics of the band of frequencies allowed to pass through the filter being determined by the physical dimensions of said stubs, the inductive reactance of each stub being varied by varying its length and also by varying its width, the skirts of the pass band response curve being determined by the depth of the stubs and by the number of elements, the limitations of the filter being limited by the limited range of individual stub reactance which occurs when the depth of a stub equals the halfbreadth of the waveguide and where the depth of a stub equals the distance between the waveguide coaxial conductor and the inside of the thick walls of the waveguide.

2. A coaxial waveguide miniature TEM microwave bandpass filter comprising:

(a) a section of ordinary size waveguide having a coaxial conductor which runs therethrough, a pair of opposite side walls of said section of waveguide being of substantial thickness with the other sides being standard thickness,

(b) at least two inductive stubs positioned in tandem along the length of said waveguide and in electrical contact with said coaxialconductor, each pair of stubs in tandem forming one element, each inductive stub consisting of a cavity formed in a thick wall of said vgaveguide section and confined within said thick wall with a stub conductor therein mounted normal to and connecting said coaxial conductor and the end of said cavity, said cavities being entirely within the outer walls of said waveguide section (0) the center of the pass band being determined by the distance between elements along the length of 4 which occurs when the depth of a stub equals the half-breadth of the waveguide and where the depth equals the distance between the waveguide coaxial conductor and the inside of the thick walls of the waveguide. 3. A coaxial waveguide miniature TEM microwave bandpass filter comprising:

(a) a section of ordinary size waveguide having a coaxial conductor which runs therethrough, a pair of opposite side walls of said section of waveguide being of substantial thickness with the other sides being of standard thickness,

(b) at least two inductive stubs positioned in tandem along the length of said Waveguide and in electrical contact with said coaxial conductor, each pair of stubs in tandem forming one element, each inductive stub consisting of a cavity formed in a thick wall of said waveguide section and confined within said thick wall with a stub conductor therein mounted normal to and connecting said coaxial conductor and the end of said cavity, said cavities being entirely within said waveguide section,

(0) the characteristics of the band of frequencies allowed to pass through the filter being determined by the physical dimension of said stubs,

(d) the skirts of the response curve for said band of frequencies allowed to pass being determined by the depth of said stubs and by the number of elements, the depth of said stubs being a determined amount less than one-quarter wavelength at the center frequency of the pass band.

(e) the center of the pass band being determined by the distance between elements along the length of the section of waveguide,

(f) a minimum of two elements being required to form a section of filter, and one additional element being required for each additional section of the filter,

(g) the spacing between elements and their depths being varied in a symmetrical arrangement about the center of said filter,

(h) the inductive reactance of each stub being varied by varying its length and also by varying its width.

References Cite-d in the file of this patent UNITED STATES PATENTS the section of waveguide, 45

(d) the characteristics of the band of frequencies al- $23 n t lowed to pass through the filter being determined 2,640,916 Espley g 2 1953 by the physical dimensions of said stubs, the in- 7 5 Dishal Ju 5 195 ductive reactance of each stub being varied by vary- 0 2,816,270 Lewis 10 1957 ing its length and also by varying its width, the 5 77 v flgg Ma 24 1960 skirts of the pass band response curve being detern y I mined by the 'depth of the stubs and by the number OTHER REFERENCES of elements, the limitations of the filter being limited Turney: Electric Filters, Pitman & Sons 1947, page by the limited range of individual stub reactance 55 161, TK 6301 T8. 

1. A COAXIAL WAVEGUIDE MINIATURE TEM MICROWAVE BANDPASS FILTER COMPRISING: (A) A SECTION OF WAVEGUIDE HAVING A COAXIAL CONDUCTOR WHICH RUNS THERETHROUGH THE WALLS OF THE BROAD SIDES OF SAID SECTION OF WAVEGUIDE BEING OF STANDARD THICKNESS AND THE WALLS OF THE NARROW SIDES OF SAID SECTION OF WAVEGUIDE BEING OF SUBSTANTIAL THICKNESS, (B) A PLURALITY OF INDUCTIVE STUBS POSITIONED IN TANDEM ALONG THE LENGTH OF SAID WAVEGUIDE AND IN ELECTRICAL CONTACT WITH SAID COAXIAL CONDUCTOR, EACH PAIR OF STUBS IN TANDEM FORMING ONE ELEMENT, EACH INDUCTIVE STUB CONSISTING OF A CAVITY FORMED IN A THICK WALL OF SAID WAVEGUIDE SECTION AND CONFINED WITHIN SAID THICK WALL WITH A STUB CONDUCTOR THEREIN MOUNTED NORMAL TO AND CONNECTING SAID COAXIAL CONDUCTOR AND THE END OF SAID CAVITY, SAID CAVITIES BEING ENTIRELY WITHIN THE OUTER WALLS OF SAID WAVEGUIDE SECTION, (C) THE CENTER OF THE PASS BAND BEING DETERMINED BY THE DISTANCE BETWEEN ELEMENTS ALONG THE LENGTH OF THE SECTION OF WAVEGUIDE, 